1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, and more particularly to a liquid crystal display employing an illuminating device having a reduced number of light sources without degradation of display quality of the display apparatus.
2. Description of the Related Art
Recent developments on the semiconductor device technology tend to make electronic devices smaller in size and lighter in weight, and such electronic devices require a new electronic display device fitting to their smaller- and lighter-designed structure. Currently, flat panel display devices having a smaller size, lighter weight, lower driving voltage and lower power consumption have been demanded for the smaller- and lighter-designed electronic devices. Among various types of the flat panel display devices, liquid crystal display (LCD) type devices are most widely used for various types of electronic devices as well as the smaller-and lighter-designed electronic devices. This is because the LCD devices generally have the advantages of light weight, small size, low driving voltage and low power consumption while being capable of displaying images with full colors.
Since an LCD device needs light to display images thereon, a light source is required to generate the light for the LCD device. The display quality and appearance of images displayed by an LCD device vary depending on characteristics of the light source. The LCD devices may be classified into a reflection type LCD, which displays images using ambient light, and a transmission type LCD, which displays images using a back light source instead of the ambient light. In the early stage of LCD development, the reflection type LCD devices were widely used, but at present the transmission type LCD devices are more widely used to display high quality images. The light source of a transmission type LCD device is usually disposed at the back of an LCD panel, and the light emitted from the light source is called ‘back light’.
As the light source of a transmission type LCD device, EL (Electro Luminescence), LED (Light Emitting Diode), CCFL (Cold Cathode Fluorescent Lamp) and HCFL (Hot Cathode Fluorescent Lamp) are widely used. In case of a portable electronic device such as personal mobile telephone (PCS phone or cellular phone) and personal digital assistant (PDA), the LED is usually used because of smaller size and lower power consumption. Especially, there has been a great increase in research on reducing the power consumption of the portable electronic devices, and recently, a research on reducing the number of LEDs has been intensively performed as one way of reducing the power consumption.
FIG. 1 is an exploded perspective view schematically showing the structure of a conventional LCD device for the small-sized products.
Referring to FIG. 1, the conventional LCD device 90 includes an LCD panel 10 for displaying an image, a backlight assembly 20 for supplying the light to the LCD panel 10, and a reflection sheet 30 for reflecting the light.
The LCD panel 10 includes an upper substrate 12 having a plurality of pixels for displaying images, a lower substrate 14 disposed under the upper substrate 12 and having a switching device 16 for electrically controlling each of the pixels, and liquid crystal (not shown) interposed between the upper and lower substrates 12 and 14, for controlling light transmittance in response to an intensity of an applied electrical field.
The backlight assembly 20 includes a light source 22 for emitting the light and a light guiding plate 24 for guiding the light emitted from the light source 22 to the LCD panel 10. The light source 22 is disposed at one side of the light guiding plate 24 and includes a light emitting diode (hereinafter referred to LED). It should be noted that the light source 22 may be disposed at each of the two opposite sides of the light guided plate 24 and include multiple LEDs. A reflection sheet 30 is disposed under the light guiding plate 24, for reflecting light leaked from the light guiding plate 24 toward the LCD panel 10 and improving light efficiency.
The switching device 16 controls the supply of electrical power to the pixels in response to image data externally provided. As a result, an electric field is formed at the position of a specific pixel between the upper and lower substrates 12 and 14, so that an aligning angle of the liquid crystal is changed in a predetermined direction. The light transmittance of the light supplied from the backlight assembly 20 is changed according as the aligning angle is changed, so that the amount of light transmitted through the liquid crystal varies according to the intensity of the electric field. The transmitted light selectively stimulates the pixels according to the image data to display an image corresponding to the image data on the LCD panel.
In the conventional LCD device, the backlight assembly 20 requires a plurality of the LEDs. However, since the LEDs are very expensive, a manufacturing cost of the LCD device inevitably increases as the number of required LEDs increases. Therefore, researches and developments have been made on the method of decreasing the number of the LEDs so as to reduce the manufacturing cost of the LCD device. Furthermore, a decrease in the number of required LEDs is also desired to reduce the power consumption of the LCD device.
However, when the number of required LEDs decreases so as to reduce the manufacturing cost and power consumption, there is a problem that luminance of the LCD device is deteriorated and the display quality of the LCD device decreases, which is a limit in decreasing the number of required LEDs.
FIG. 2A is a schematic view for explaining the light supply by a conventional illuminating device adopted as a component of backlight assembly of the conventional LCD device, and FIG. 2B is a schematic view explaining the light supply when the number of the LEDs in FIG. 2A is reduced into three.
Generally, when a luminous body of a point light source such as an LED emits light to a light guiding plate 24, the quantity of light at the angles of ±60° with respect to an imaginary line vertically connected between the light source (e.g., LED) and the light guiding plate 24 is about half of the quantity of light at the angle of 0°. The light emitted from the point light source travels to and reaches at a certain area of the light guiding plate 24, ranging from the right angle of 60° in the clockwise direction (hereinafter, referred to as “+60°”) to the left angle of 60° in the counterclockwise direction (hereinafter, referred to as “−60°”) with respect to the imaginary line vertically connected between the light source and the light guiding plate 24. When the light is radiated, the quantity of marginal light 22a which is traveling to the light guiding plate at the angle of +60° or −60°, is about half of that of central light 22b which is traveling to the light guiding plate 24 along a front direction at the angle of 0°. Therefore, the light quantity at a marginal region {circle around (2)} of the light guiding plate 24 at which the marginal light 22a arrives is about half of the light quantity at a central region {circle around (1)} of the light guiding plate 24 at which the central light 22b arrives. Consequently, as shown in FIG. 2A, LEDs in the light source are each installed such that the adjacent marginal lights arrive at the same marginal region of the light guiding plate 24 to prevent a drop-off of the luminance due to the reduction of light quantity at the marginal region {circle around (2)} of the light guiding plate 24.
When the conventional illuminating device is adopted as a light source of the backlight assembly of the LCD device, the display quality of the LCD device is deteriorated due to a quantity difference in the lights arriving at different regions of the light guiding plate 24. In case that the number of the LEDs decreases, the light generated from the light source 22 may not arrive at some portion of the light guiding plate 24. As shown in FIG. 2B, since a distance between the adjacent LEDs increases as the number of the LEDs decreases, the light may not arrive at a portion of the light guiding plate 24, which is corresponding to a region between marginal areas at which the adjacent marginal lights arrive, respectively. Therefore, in FIG. 2B, portion “B” of the light guiding plate 24 corresponding to an emitting region of each LED is more bright than portion “A” of the light guiding plate 24 corresponding to a non-emitting region of each LED between the adjacent marginal areas. Accordingly, a bright portion and a dark portion are alternately shown on the LCD panel 10 disposed on the light guiding plate 24, which deteriorates the display quality.
In order to prevent the display quality of the LCD device from decreasing, various developments have been performed on the modified light guiding plate. For example, there have been provided a light guiding plate including a concave portion or a convex portion to serve as a lens, which is disposed on the incident surface of the light guiding plate onto which the light is incident, a light guiding plate including a plurality of irregularities for scattering the light, or a light guiding plate including a speculum structure thereon for reflecting the light. However, the above-modified light guiding plates have an inevitable limit such as a luminance drop. This is because the light generated from a light source is incident into the light guiding plate 24 after being reflected on the modified portion of the light guiding plate 24.